dillon1000/react

Bug repro for unobserved aliased mutation w phi

I found this while working to ensure that we always lower all operands to 
temporaries. This works: 

```javascript 

// the whole computation of x is memoized in one block, bc of the mutation after 
the phi 

let x; 

if (cond) { 

x = someObj(); 

} else { 

x = someObj(); 

} 

mutate(x); 

``` 

However, if you alias either of the operands, we lose the mutation: 

```javascript 

let x; 

if (cond) { 

const y = someObj(); // OOPS this gets independently memoized 

x = y; 

} else { 

x = someObj(); 

} 

mutate(x); 

``` 

The core issue is that InferMutableRanges does not take into account mutation of 
phis. ~~My first thought is that we need an additional, outer fixpoint iteration 
loop to flow mutation back "up" to phi operands~~ 

edit: there was a much easier fix, we need to alias phi operands and phi id 
within the existing fixpoint iteration. See follow-up PR which fixes.